Preparation And Electrochemical Performance Of Manganese-based Cathode Materials For Aqueous Zinc-ion Batteries

Aqueous zinc-ion batteries show promising application in the future large-scale energy storage due to their high safety performance,low cost,eco-friendly and other advantages.However,the electrochemical performance of aqueous zinc-ion batteries cannot satisfy the needs of practical applications.The cathode materials,as an important part of aqueous zinc-ion batteries,directly affects the overall performance of the battery.Manganese oxides are regarded as a promising class of cathode materials,which have received extensive attention because of their low price,high operating voltage,considerable energy density,and so on.But there are many problems to be solved,such as sluggish electrical conductivity,slow diffusion kinetics of Zn²⁺,dissolution of electrode materials,irreversible phase transition,severe volume expansion and structural collapse during cycling.These shortcomings lead to rapid decay of their rate and cycling performance.Based on the above considerations,in this thesis,we adopt the strategies of defect engineering,interfacial modifications,cations doping and the construction of composite materials to prepare high-performance manganese oxide cathode materials.Besides,their electrochemical properties and reaction mechanism were explored.The main conclusions are as follows:1.The composite cathode material（DMOC）of manganese-deficient Mn₃O₄nanoparticles（DMO）grown in situ carbon nanotubes（CNTs）were prepared.Electrochemical tests and theoretical calculations demonstrate that Mn defects not only enhance the intrinsic conductivity of material,but also create more active sites for the storage of Zn²⁺,thus promoting electrochemical reaction kinetics.Moreover,the introduction of CNTs is favorable for exposing abundant Mn defects for electrochemical reactions and enhancing the utilization of Mn defects.Benefitting for the unique structural advantages,Zn//DMOC batteries show a high discharge capacity of 420.6 m A h g^–1,and the capacity retention is higher than 80%after 2800 cycles under the current density of 2.0 A g^–1,demonstrating the excellent cell performance.The results of electrochemical kinetic tests and the structural and morphological analyses during cycling process demonstrate that DMOC possesses a fast diffusion kinetics of Zn²⁺and excellent structural stability.Moreover,the pouch battery assembled with DMOC cathode display good flexible and delivers a long cycle life.2.We constructed the porous composite of carbon-containing Mn O（MOC）and nitrogen-doped graphene aerogel（NGA）,donated as MOC@NGA by in-situ coprecipitation and high-temperature calcination process.The research results reveal that the abundant Mn-O-C and Mn-N interfacial chemical bonds in MOC@NGA can reduce the interfacial gap of the composite.Moreover,the interfacial chemical bonds,as the electron transport highway,accelerate the electron transfer of the composite,reduce the interfacial charge transfer resistance and enhance the Zn²⁺reaction kinetics.In addition,the introduction of 3D porous NGA increases the specific surface area of the composite,which helps expose more active sites and shorten the ion diffusion distance.Therefore,the MOC@NGA cathode exhibits a high specific capacity of 270m A h g^-1 at a current density of 0.1 A g^-1,and the energy density is achieved 370.2 Wh kg^-1.And the discharge capacity remains 151.6 m A h g^-1 after 2000 cycles at a current density of 1.0 A g^-1.During the cycling process,the MOC@NGA is partially converted into Mn O₂,and both Zn²⁺and H⁺participate in the electrochemical reaction.Importantly,the interfacial chemical bond can remain stable,which is beneficial for improving the rate performance and cycle stability.In addition,the flexible quasi-solid-state device assembled with MOC@NGA electrode achieves a discharge specific capacity of 98.5 m A h g^-1 after 1000 cycles at 1.0 A g^-1.3.The Co^2+/3+doped hollow porous Mn₂O₃（HP-0.04CMO）material were prepared and by microwave rapid heating and high-temperature pyrolysis process,and it can serve as the cathode of aqueous zinc-ion batteries.Benefiting for the synergistic effect of suitable Co doping and hollow porous structure,Zn//HP-0.04CMO batteries deliver the specific discharge capacities of 358.8 and 180.5 m A h g^–1 at current densities of 0.1 and 3.0 A g^–1,demonstrating excellent rate performance.Impressively,the cathode exhibits durable cycling performance.Its specific capacity still maintains 95.6m A g^–1 after 6600 cycles at a current density of 1.0 A g^–1,which is superior to most reported manganese-based cathode materials.In addition,the quasi-solid Zn//HP-0.04CMO device also displays good electrochemical performance.Reaction kinetic analysis and energy storage mechanism exploration reveal that the excellent electrochemical performance of HP-0.04CMO is mainly due to the fact that the doped Co can improve the conductivity of the material and participate in the redox reaction process,thus providing additional discharge specific capacity.Moreover,the doped Co also serve as"interlayer pillar"for the charging product of Mn O₂,which can enlarge the interlayer spacing and improve the diffusion kinetics of Zn²⁺.In addition,the unique hollow porous structure of HP-0.04CMO can not only shorten the ions diffusion distance,but also release the internal structural stress caused by the repeated de-intercalation of Zn²⁺ions and improve the structural stability of cathode.The research results obtained in this thesis provide new ideas for the development of high-performance manganese-based cathodes for aqueous zinc-ion batteries,and the results also deepen the cognition and understanding of the energy storage mechanism for manganese-based oxides.